My homework problem is this exact one.

https://www.coursehero.com/file/25994701/HW4-Solutions/

Would it be possible to get it?

I am understanding it, but I keep getting confused on where to model the concentrations.

"1. (20 pts) Mixing Problem You have a 5 gallon fresh water fish tank, which you have not cleaned in a while, and the water looks pretty cloudy (it should be clear). You take a sample and determine that there is 50 grams of pollutant per gallon of water. A healthy tank should have less than 10 grams of pollutant per gallon and you decide your goal is to get to 5 g of pollutant per gallon. Initially there is 4 gallons of water in the tank. To not shock your poor fish, you decide to dilute the pollutant by adding 1 gallon of clean water every 30 minutes and drain the mixed water at the same rate. (a) At this rate of putting water in and taking water out at the same rate, how long does it take to reach a pollutant level of 5 g/gal? Be sure to define all variables in a table! (b) Note: you must get the solution to part (a) correct to get any credit for part (b). Being confident in your math skills, you decide you can afford to leave for 3 hours to watch a WSU basketball game with some friends. However, you do not realize you are are not thinking like an engineer (!). 1 hour after you leave the outlet valve gets partially clogged and the rate out is reduced from 1 gallon every 30 minutes to 1 gallon every hour. Let t = 0 be the time at which flushing begins. i. When is the tank full? You return after 3 hours. How much water is on the floor? ii. Note: you must get the solution to part (b.i) correct to get any credit for part (bii). What is the mass of pollutant in the tank when the tank first fills? iii. Note: you must get the solution to part (b.ii) correct to get any credit for part (biii). After the tank is full, assume the flow rate in is 1 gallon of water every hour (the same as the flow rate out). What is the mass of pollutant still in the tank when you return after 4 hours? Is it still above the maximum (equivalent to 10 g/gallon)?"